Your child entered the social world when he started his schooling. He met new people his age and started interacting with them. This is indeed a good thing for the kid to observe and learn human nature in a better way. But you need to make sure the kid is capable of understanding emotions.
What you need to know
Most kids can not catch various emotions which backfires and makes them insensitive. Teach the kid about happiness, sorrow, despair, anger and apology. You can demonstrate this at home by role playing with the kid. You can act as if you are sad and observe how the kid treats you.
When the child does something wrong, then ask him to apologise for it. You might have to be strict at times but make sure it proceeds in a positive manner. Understanding emotions would help your child address and help his friends.